1Linux Magic System Request Key Hacks 2==================================== 3 4Documentation for sysrq.c 5 6What is the magic SysRq key? 7~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 8 9It is a 'magical' key combo you can hit which the kernel will respond to 10regardless of whatever else it is doing, unless it is completely locked up. 11 12How do I enable the magic SysRq key? 13~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 14 15You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when 16configuring the kernel. When running a kernel with SysRq compiled in, 17/proc/sys/kernel/sysrq controls the functions allowed to be invoked via 18the SysRq key. The default value in this file is set by the 19CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults 20to 1. Here is the list of possible values in /proc/sys/kernel/sysrq: 21 22 - 0 - disable sysrq completely 23 - 1 - enable all functions of sysrq 24 - >1 - bitmask of allowed sysrq functions (see below for detailed function 25 description):: 26 27 2 = 0x2 - enable control of console logging level 28 4 = 0x4 - enable control of keyboard (SAK, unraw) 29 8 = 0x8 - enable debugging dumps of processes etc. 30 16 = 0x10 - enable sync command 31 32 = 0x20 - enable remount read-only 32 64 = 0x40 - enable signalling of processes (term, kill, oom-kill) 33 128 = 0x80 - allow reboot/poweroff 34 256 = 0x100 - allow nicing of all RT tasks 35 36You can set the value in the file by the following command:: 37 38 echo "number" >/proc/sys/kernel/sysrq 39 40The number may be written here either as decimal or as hexadecimal 41with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be 42written in hexadecimal. 43 44Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation 45via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is 46always allowed (by a user with admin privileges). 47 48How do I use the magic SysRq key? 49~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 50 51On x86 52 You press the key combo :kbd:`ALT-SysRq-<command key>`. 53 54 .. note:: 55 Some 56 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is 57 also known as the 'Print Screen' key. Also some keyboards cannot 58 handle so many keys being pressed at the same time, so you might 59 have better luck with press :kbd:`Alt`, press :kbd:`SysRq`, 60 release :kbd:`SysRq`, press :kbd:`<command key>`, release everything. 61 62On SPARC 63 You press :kbd:`ALT-STOP-<command key>`, I believe. 64 65On the serial console (PC style standard serial ports only) 66 You send a ``BREAK``, then within 5 seconds a command key. Sending 67 ``BREAK`` twice is interpreted as a normal BREAK. 68 69On PowerPC 70 Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`. 71 :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice. 72 73On other 74 If you know of the key combos for other architectures, please 75 let me know so I can add them to this section. 76 77On all 78 Write a character to /proc/sysrq-trigger. e.g.:: 79 80 echo t > /proc/sysrq-trigger 81 82The :kbd:`<command key>` is case sensitive. 83 84What are the 'command' keys? 85~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 86 87=========== =================================================================== 88Command Function 89=========== =================================================================== 90``b`` Will immediately reboot the system without syncing or unmounting 91 your disks. 92 93``c`` Will perform a system crash and a crashdump will be taken 94 if configured. 95 96``d`` Shows all locks that are held. 97 98``e`` Send a SIGTERM to all processes, except for init. 99 100``f`` Will call the oom killer to kill a memory hog process, but do not 101 panic if nothing can be killed. 102 103``g`` Used by kgdb (kernel debugger) 104 105``h`` Will display help (actually any other key than those listed 106 here will display help. but ``h`` is easy to remember :-) 107 108``i`` Send a SIGKILL to all processes, except for init. 109 110``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. 111 112``k`` Secure Access Key (SAK) Kills all programs on the current virtual 113 console. NOTE: See important comments below in SAK section. 114 115``l`` Shows a stack backtrace for all active CPUs. 116 117``m`` Will dump current memory info to your console. 118 119``n`` Used to make RT tasks nice-able 120 121``o`` Will shut your system off (if configured and supported). 122 123``p`` Will dump the current registers and flags to your console. 124 125``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular 126 timer_list timers) and detailed information about all 127 clockevent devices. 128 129``r`` Turns off keyboard raw mode and sets it to XLATE. 130 131``s`` Will attempt to sync all mounted filesystems. 132 133``t`` Will dump a list of current tasks and their information to your 134 console. 135 136``u`` Will attempt to remount all mounted filesystems read-only. 137 138``v`` Forcefully restores framebuffer console 139``v`` Causes ETM buffer dump [ARM-specific] 140 141``w`` Dumps tasks that are in uninterruptable (blocked) state. 142 143``x`` Used by xmon interface on ppc/powerpc platforms. 144 Show global PMU Registers on sparc64. 145 Dump all TLB entries on MIPS. 146 147``y`` Show global CPU Registers [SPARC-64 specific] 148 149``z`` Dump the ftrace buffer 150 151``0``-``9`` Sets the console log level, controlling which kernel messages 152 will be printed to your console. (``0``, for example would make 153 it so that only emergency messages like PANICs or OOPSes would 154 make it to your console.) 155=========== =================================================================== 156 157Okay, so what can I use them for? 158~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 159 160Well, unraw(r) is very handy when your X server or a svgalib program crashes. 161 162sak(k) (Secure Access Key) is useful when you want to be sure there is no 163trojan program running at console which could grab your password 164when you would try to login. It will kill all programs on given console, 165thus letting you make sure that the login prompt you see is actually 166the one from init, not some trojan program. 167 168.. important:: 169 170 In its true form it is not a true SAK like the one in a 171 c2 compliant system, and it should not be mistaken as 172 such. 173 174It seems others find it useful as (System Attention Key) which is 175useful when you want to exit a program that will not let you switch consoles. 176(For example, X or a svgalib program.) 177 178``reboot(b)`` is good when you're unable to shut down, it is an equivalent 179of pressing the "reset" button. 180 181``crash(c)`` can be used to manually trigger a crashdump when the system is hung. 182Note that this just triggers a crash if there is no dump mechanism available. 183 184``sync(s)`` is handy before yanking removable medium or after using a rescue 185shell that provides no graceful shutdown -- it will ensure your data is 186safely written to the disk. Note that the sync hasn't taken place until you see 187the "OK" and "Done" appear on the screen. 188 189``umount(u)`` can be used to mark filesystems as properly unmounted. From the 190running system's point of view, they will be remounted read-only. The remount 191isn't complete until you see the "OK" and "Done" message appear on the screen. 192 193The loglevels ``0``-``9`` are useful when your console is being flooded with 194kernel messages you do not want to see. Selecting ``0`` will prevent all but 195the most urgent kernel messages from reaching your console. (They will 196still be logged if syslogd/klogd are alive, though.) 197 198``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process 199you are unable to kill any other way, especially if it's spawning other 200processes. 201 202"just thaw ``it(j)``" is useful if your system becomes unresponsive due to a 203frozen (probably root) filesystem via the FIFREEZE ioctl. 204 205Sometimes SysRq seems to get 'stuck' after using it, what can I do? 206~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 207 208That happens to me, also. I've found that tapping shift, alt, and control 209on both sides of the keyboard, and hitting an invalid sysrq sequence again 210will fix the problem. (i.e., something like :kbd:`alt-sysrq-z`). Switching to 211another virtual console (:kbd:`ALT+Fn`) and then back again should also help. 212 213I hit SysRq, but nothing seems to happen, what's wrong? 214~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 215 216There are some keyboards that produce a different keycode for SysRq than the 217pre-defined value of 99 218(see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or 219which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find 220an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map 221this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's 222probably best to put this command in a boot script. Oh, and by the way, you 223exit ``showkey`` by not typing anything for ten seconds. 224 225I want to add SysRQ key events to a module, how does it work? 226~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 227 228In order to register a basic function with the table, you must first include 229the header ``include/linux/sysrq.h``, this will define everything else you need. 230Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key 231handler function you will use, B) a help_msg string, that will print when SysRQ 232prints help, and C) an action_msg string, that will print right before your 233handler is called. Your handler must conform to the prototype in 'sysrq.h'. 234 235After the ``sysrq_key_op`` is created, you can call the kernel function 236``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will 237register the operation pointed to by ``op_p`` at table key 'key', 238if that slot in the table is blank. At module unload time, you must call 239the function ``unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)``, 240which will remove the key op pointed to by 'op_p' from the key 'key', if and 241only if it is currently registered in that slot. This is in case the slot has 242been overwritten since you registered it. 243 244The Magic SysRQ system works by registering key operations against a key op 245lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has 246a number of operations registered into it at compile time, but is mutable, 247and 2 functions are exported for interface to it:: 248 249 register_sysrq_key and unregister_sysrq_key. 250 251Of course, never ever leave an invalid pointer in the table. I.e., when 252your module that called register_sysrq_key() exits, it must call 253unregister_sysrq_key() to clean up the sysrq key table entry that it used. 254Null pointers in the table are always safe. :) 255 256If for some reason you feel the need to call the handle_sysrq function from 257within a function called by handle_sysrq, you must be aware that you are in 258a lock (you are also in an interrupt handler, which means don't sleep!), so 259you must call ``__handle_sysrq_nolock`` instead. 260 261When I hit a SysRq key combination only the header appears on the console? 262~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 263 264Sysrq output is subject to the same console loglevel control as all 265other console output. This means that if the kernel was booted 'quiet' 266as is common on distro kernels the output may not appear on the actual 267console, even though it will appear in the dmesg buffer, and be accessible 268via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific 269exception the header line from the sysrq command is passed to all console 270consumers as if the current loglevel was maximum. If only the header 271is emitted it is almost certain that the kernel loglevel is too low. 272Should you require the output on the console channel then you will need 273to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or:: 274 275 echo 8 > /proc/sysrq-trigger 276 277Remember to return the loglevel to normal after triggering the sysrq 278command you are interested in. 279 280I have more questions, who can I ask? 281~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 282 283Just ask them on the linux-kernel mailing list: 284 linux-kernel@vger.kernel.org 285 286Credits 287~~~~~~~ 288 289- Written by Mydraal <vulpyne@vulpyne.net> 290- Updated by Adam Sulmicki <adam@cfar.umd.edu> 291- Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 292- Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> 293